Composite and component video and audio equipment frequently utilizes an external reference signal for synchronization. The use of a reference signal is particularly important in the case of digital video and audio signals, where the reference signal serves to avoid buffer management problems, manage jitter and maintain a defined relationship between video and audio signals.
A reference signal is produced by a sync generator which contains a crystal-controlled highly stable oscillator that generates a signal of the same frequency as an information signal, for example a video or audio signal, but with precise timing signals that are not subject to frequency drift or time base errors which might affect the information signal. For example, in the case of a colour video signal the sync generator generates horizontal sync, vertical sync, colour reference, burst flag and blanking pulses, typically as a colour black reference signal, at precise intervals corresponding to the nominal specifications for the format of the particular signal being processed. North American colour television signals conform to the NTSC standard with a colour subcarrier frequency of 3.579545 MHz to produce 60 fields per second, or 30 frames of 525 interlaced horizontal lines per frame, while most European colour television signals conform to the PAL standard with a colour subcarrier frequency of 4.43361875 MHz to produce 50 fields per second, or 25 frames of 625 interlaced horizontal lines per frame.
There are many situations in which signals from sources at different locations must be integrated or combined, for example a video signal and its associated audio signal, where for example an NTSC video signal received from a satellite feed or microwave repeater relay network may have its associated audio signal transmitted to the broadcast facility over land lines. In other cases a remote video feed may be integrated with a local video signal, for example in a news broadcast or a live interview between the broadcast facility and a remote location.
In a typical broadcast facility video signals originating from within the broadcast facility are genlocked to the reference signal generated by a master generator, which provides a common timing reference for aligning the various signals, as is well known. Frame synchronizers compare the timing of sync, burst flag etc. pulses in the reference signal with the timing of corresponding pulses in the various video signals, and where a timing difference exceeds a preselected tolerance the frame synchronizer either repeats a frame or drops a frame to compensate for the phase offset of the information signal.
However, a problem arises when it becomes necessary to combine information signals originating from sources at different locations. The lengths of the signal paths may differ, for example where a video signal is relayed via satellite and its associated audio signal is transmitted over land lines, which causes a delay differential between the signals, or independent video signals containing information relating to a common event, for example a local interviewer interviewing a person at remote location, or a panel discussion between panelists in different cities, will arrive at the broadcast facility at different times if the signal paths differ. Each remote signal source may be locked to an independent reference signal, which may not be in precise alignment with other unrelated reference signals or the reference signal used by the broadcast facility.
Additional problems arise in the case of signals having different formats, for example an NTSC video signal and a PAL video signal. Video signals must be in the same format in order to be combined or integrated into a single broadcast, and the conversion of one signal to the format of another signal causes a delay in the converted signal which results in, or exacerbates, a delay differential between the signals, and the reference signals associated with these video signals are incompatible with one another.
Devices for synchronizing frames between signals having the same format, for example two NTSC video signals, are known. However such devices operate by aligning frame boundaries, ie. aligning line 1, field 1 of one video signal with line 1, field 1 of another video signal in the same format, or aligning signals to a reference signal, to the nearest frame boundary. This is a much more complex problem in the case of signals having different formats. Moreover, frequently the extent of the delay differential between signals originating from different locations extends to many frames, and there has heretofore been no satisfactory method for aligning such signals by information content.
Using conventional methods signals from sources at different locations can often be frame-aligned, but human intervention (largely reliant upon the experience of skilled technicians and a considerable amount of guesswork) has been necessary to content-align multiple signals. Thus, a video signal and its associated audio signal must be aligned to avoid so-called"lip-sync" errors, or two video signals which contain related information such as an interactive conversation between persons at disparate locations must be aligned so that the resulting integration appears to flow without unnatural pauses or delays.